Furnace control



L. W. POLL-OCK FURNACE CONTROL Dec. 3, 1963 2 Sheets-Sheet 1 Filed Nov. 21, 1962 FEED FUEL

EFFIuEN TEMPERATURE DIFFERENCE TEMPERATURE RECORDER HEATED PASSAGEWAY SU MER RECORDER-l INVENTOR. L. w. POLLOCK TEMPERATURE DIFFERENCE SIGNAL BY v gd M W Y 9 A r TORNEKS CONTROLLER FUNCTION GENERATOR United States Patent Petroleum Company, a corporation of Delaware Filled Nov. 21, 1962, Ser. No. 256,974) 4 Claims. (Cl. 236-20) This invention relates to furnace control. In one of its aspects, the invention relates to a method permitting optunum firing of a tube in a heater by detecting the average temperature of medium being heated within the tube, determining the allowable temperature differential therefrom, determining the actual temperature differential, comparing these two differentials and thereby mampulating the flow of fuel gas to a burner or a group of burners which supplies heat to the tube so that the actual temperature differential will be substantialy equal to the allowable temperature differential. In another of its aspects, the invention relates to an apparatus adapted for optimally firing a tube in which a medium is being heated which comprises means for detecting the temperatures at points spaced along said tube, means for detecting the average temperature of medium heated within the tube, means for transmitting a signal corresponding to said average temperature, a function-generator means to which the signal is transmitted which in turn transmits a signal corresponding to a perimssible temperature differential, means for transmitting the difference in temperature between said points, a controller means adapted to compare said temperature difference with said permissible difference for said tube determined from the average temperature and to transmit a signal corresponding to the corrected heat input to said tube to the source of heat thereto so that the temperature difference will become substantially equal to the permissible temperature difference.

In the cracking of light hydrocarbons to produce olefins in tube-type radiant furnaces, such as a Selas furnace, the factor limiting the capacity of the funrace is usually the allowable tube wall temperature. This temperature is a function of tube size, cracked gas rate, cracked gas temperature, and the conversion rate. As the cracked gas temperature increases, more of the heat input is used in the endothermic reaction and less is available as sensible heat to increase the gas temperature. Also, as the cracked gas temperature increases, the allowable heat flux decreases and, as a result, the allowable temperature rise per tube decreases through the furnace. This invention utilizes the rise in temperature per tube by detecting this rise in temperature as later explained.

An object of this invention is to provide a method for operating optimally a furnace or heater. Another object of the invention is to provide an apparatus Wherewith optimal operation of tubes in a heating zone can be accomplished. A further object of this invention is to provide method and apparatus for cracking a gas in a furnace in which the limiting factor ,on the capacity of the furnace essentially is the tube wall temperature of each of the several tubes in the furnace. A further object of the invention is to provide for automatic control of the heat supplied to a tube within a heater or furnace in a manner such that maximum heat, consistent with tube safety, is transmited through each tube of the furnace.

Other aspects, objects, and the several advantages of the invention are apparent from this disclosure, thed rawing and the appended claims.

According to the present invention, there are provided method and means in the operation of a furnace in which tubes are heated by a fuel burned in their proximity which comprise detecting the temperature rise in a tube, comparing this rise with a predetermined permisddlzfidh Patented Dec. 3, 1963 "ice sible rise and controlling the burner in proximity to said tube in accordance with said comparison.

It will be obvious to one skilled in the art in possession of this disclosure, having studied the same, that the gas temperature rise through several tubes located in proximity of one or more burners can be determined rather than simply the temperature rise through a tube or through each tube.

Briefly described, the apparatus of the invention as it is applied to the control of a burner or burners heating a tube in a furnace comprises essentially a differential temperature recorder which puts out a signal corresponding to the difference between the temperatures detected at the ends of a tube or between any two points along the heating passageway of a heater, an electronic operational amplifier with multiple input arrangements which functions as a summer which puts out a signal corresponding to the average gas temperature within said tube or tubes or passageway, an electronic function-generator which is supplied with a correlation of the temperature of the medium being heated within the tube and allowable AT or difference in temperature for the tube for the particular feed stock being processed, at temperature controller adapted to control a burner or burners affecting the temperature of said tube or tubes or passageway, the function-generator being fed a signal from the electronic operational amplifier which in turn receives the individually measured tube temperature signals, the thuscomputed allowable differential temperature and the actual measured differential temperature being employed by a controller as set point and measurement, respectively, so as to manipulate the flow of fuel gas, and thereby the heat input, so that the allowable AT is achieved by the process.

In the drawing, FIGURE 1 is a diagrammatic showing of the arrangement of the component parts of the apparatus of the invention. FIGURE 2 illustrates, in vertical cross-section and diagrammatically, a furnace to which the method and apparatus of the invention are applicable and FIGURE 3 shows the allowable difieronce per tube vs. the average temperature of the gas in said tube for three feed stocks, namely, reading from bottom toward the top, ethane, propane, and butane, respectively, for a given furnace, total feed, and steamto-hydrocarbon ratio.

Referring now to FIGURE 1, the invention is described as it can be applied to three different feed stocks, namely, ethane, propane, and butane, as these are cracked at high temperatures to produce olefins, for example,

ethylene and propylene and butylenes.

The gas temperature rise through tube 7 is obtained by transmitting electrical signals, which are proportional to the gas temperatures as sensed by thermocouples, etc., as output from temperature recorders 1 and 2 (such as Brown Electronik Recorders, Series 152Xl3) to a temperature difference recorder 3 which can be of the same type. The output signals from temperature recorders 1 and 2 are also transmitted through coefiicient scaling potentiometers 4 and 5 such as Electronic Associates, Inc. Fotentiometer, Type 16-7M, which multiply the voltages by a 0.5 scaling factor. The outputs from the potentiometers are transmitted to a standard electronic operational amplifier 6 with multiple input arrangements to function as a summer such as an Electronic Associate Ampiifier, Type iii-16H. The output from the summer is equal to the arithmetic average gas temperature in tube 7. The output from the amplifier is transmitted to a standard electronic function-generator 9 such as an Electronic Associates, Type 1616B. This functiongenerator is supplied with the correlation of the average gas temperature vs. the allowable AT/tube for the particular feed stock being processed, as shown by the curves depicted in FIGURE 3.

Function-generator 9 determines the allowable differential temperature from the input value of average gas temperature by means of the above-mentioned correlation, and transmits this signal to standard electronic temperature controller 8 wherein it is employed as the set point. The signal transmitted from aforementioned temperature difference recorder 3, which is representative of the actual differential temperature existing in the process, is received by controller 8 as the measurement signal. Controller 8 compares these signals and produces a corrective signal which it transmits to control valve 10 so as to manipulate the fuel gas flow and thereby the heat input to radiant burner or burners 21 that are largely responsible for the heat input to the tubes being considered to force the actual AT to achieve and maintain the value of the allowable AT. Burners 22-26 can be controlled similarly to burner 21.

The calculated data necessary to provide the curves depicted in FIGURE 3 are presented in Table I for the four tubes nearest the outlet in the radiant section of furnace 20. The conversion level, as well as feed rate and steam/hydrocarbon ratio, has an effect on the calculated curves. The allowable AT (g gas) per tube is calculated by multiplying AT by the allowable AT (s skin) over actual AT For the tube numbered 1 in the Ethane Cracking tabulation, it is noted that the AT of the gas per tube exceeds the allowable AT per tube as obtained from the graph. In this instance, the function-generator, receiving the value of gas temperature (1531 F.) determines the allowable AT (9.4 F.) therefrom. This latter value is compared with the actual AT (11 F.) in controller 8 and as the result, the control signal to valve 10 is changed, causing a decrease in the rate of fuel to the burners and the consequent heat input to the tube so that the actual measured AT is reduced to the set point value of 9.4 F. In the other examples (except tube No. 2 of the Ethane Cracking tabulation) in Table I, the allowable AT s are above the actual AT s. For these conditions, the temperature controller signals for more heat input to the respective tubes involved; thereby increasing the gas heating and cracking which produces greater per-pass yields of valuable olefinic products without overheating the stainless steel tubes.

Thus, the invention permits maximum heat input and use of the tubes without endangering the tubes. In other words, optimum firing of the furnace is obtained.

TABLE I CALCULATED FURNACE CONDITIONS FOR ETHANE CRACKING 0.43 MOL STEAM/MOL ETHANE FEED Tube Gas AT: Skin Allowable Number Temp, per Temp. ATs Allowable AT]; per from F. Tube (s), F. Al, Tube Outlet (graph) CALCULATED FURNACE CONDITIONS CRACKING 0.61

FOR PROPANE MOL STEAM/MOL PROPANE FEED CALCULATED FURNACE CONDITIONS FOR BUTANE CRACKING 0.69 MOL STEAM/MOL BUTANE FEED 4 The following tabulation will be helpful in the art reading this disclosure:

to one skilled Unit Brand Type Temperature Recorder BIOWl Electronik Re- Series 152X13.

(1 cor er. Temperature Recorder Brown Electronic Con- Series 1521113.

troller (8). trolle Type 16-7M. Coeflicient Scaling Potenr. Electronic Assoeiate or Mid-Century Instru- In the foregoing table, the digits within the parentheses represent digits appearing in FIGURE 1.

EXAMPLE Feed stock, as shown above, is fed to a tube-type cracking furnace similar to that depicted in FIGURE 2 having fifteen schedule 40 convection tubes and eighteen 4 /2 inch ID. x 5 inch O.D. radiant tubes, all tubes 36 feet in length and constructed of Type 302B stainless steel. The radiant section is equipped with a total of six burner assemblies, each assembly positioned to heat three of the tubes. Each burner row, for every three radiant tubes, is individually controlled.

Run No 1 2 3 Feed stock Ethane Propane Butane Total hydrocarbon rate, lb./hr 5, 515 7, 148 7, 840 Total steam rate, lb./hr 1, 435 1,773 1,680 Steam/hydrocarbon,rnol ratio. 0. 43 O. 01 0. 69 Conversion, percent 65 92 96 Elliuent furnace pressure, p.s.i. 25 25 25 Limiting effluent velocity, ft./scc 800 850 850 Limiting skin temperature of Type 3021) stainless steel tubes, F 1, 600 1, (S00 1, 600

This application is a refile of my application Serial Number 1,185, filed January 8, 1960, now abandoned.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, drawing and the appended claims to the invention, the essence of which is that actual and computed temperature differences across a tube are compared, obtaining a control signal employed to adjust fuel input to a burner affecting heat input to said tube; that a method for controlling heat input to a heated passageway which comprises determining temperature difference between tWo points spaced apart in said passageway, controlling heat supplied to said passageway responsive to the difference in temperature which has been determined and modifying said controlling responsive to a comparison of the actual temperature difference between said points with a permissible temperature difference between said points for the medium being heated, the permissible temperature difference being a function of the temperature of the medium between said two points, has been provided; and that an apparatus, as described, has also been provided.

I claim:

1. A method of controlling the heat flowing from a burner to a passageway in a heater which comprises detecting at each of two points along said passageway the respective temperatures at said points of a medium being heated within said passageway, determining the difference between said temperatures, controlling heat supplied to said passageway between said points responsive to the difference bet-ween said temperatures, computing from said temperatures a predetermined permissible temperature rise of the medium in said passageway between said points at the average temperature of said medium in said passageway between said two points, and adjusting said controlling of heat supplied to said passageway between said two points responsive to said computing to maintain the said temperature rise between said two points at a desired value.

2. A method of controlling the temperature rise of a medium flowing bet-ween two points in a heated passageway which comprises transmitting electrical signals proportional to the respective temperatures of said medium at each of said two points to a temperatu re-difierentialrecorder zone and computing the dilierence between said temperatures, also transmitting said signals to a summing zone yielding an output signal proportional to the arithmetic average of said temperatures, controlling by means of a temperature-controlling zone heat input to said passageway between said points responsive to said difference by feeding a signal proportional to said difference to said temperature controlling zone; in a function-generator zone receiving from said summing zone a signal resulting herein from said electrical signals, causing said functiongenerator zone to compare said last signal with a correlation of gas temperature vs. the allowable temperature difference for the passageway for the medium being processed in said passageway, thereby obtaining an allowable temperature difference signal, and feeding said allowable temperature difference signal to said temperature controlling zone to reset the same.

3. A method for controlling heat input to a heated passageway which comprises determining temperature difference between two points spaced apart in said passageway, controlling heat supplied to said passageway responsive to said temperature difference and modifying said controlling responsive to a comparison of said temperature diiference, bet-ween said points, with a temperature difference which is permissible between said points for the medium being heated, at the temperature of the medium between said two points.

4. An apparatus for controlling the heat input to a heated passageway which comprises a temperature-recording means at each of two spaced points along said passageway to determine the temperatures within said passageway at said points, a temperature-diflerence-recording means capable of transmitting a signal representative of temperature difference determined thereby, means for transmitting from said temperatureaecording means signals respectively representatively of the determined temperatures, means for determining and yielding a signal representafive of the arithmetic average of said temperatures, means for transmitting said signals respectively representative of said determined temperatures to said means for determining and yielding a signal representative of said arithmetic aver-age, means for controlling heat input to said passageway, means for transmitting the temperature difference signal to said means for controlling, a means for generating a function adapted to compare a correlation of allowable temperature difference between said points set up therein with a signal transmtited from said arithmetic average determining means and to yield a signal representative of a permissible temperature difference in said passageway and means for transmitting the last signal to reset said means for controlling heat input to said passageway.

Borden et al. Oct. 1, 1935 Maienhein Jan. 10, 1950 

3. A METHOD FOR CONTROLLING HEAT INPUT TO A HEATED PASSAGEWAY WHICH COMPRISES DETERMINING TEMPERATURE DIFFERENCE BETWEEN TWO POINTS SPACED APART IN SAID PASSAGEWAY, CONTROLLING HEAT SUPPLIED TO SAID PASSAGEWAY RESPONSIVE TO SAID TEMPERATURE DIFFERENCE AND MODIFYING SAID CONTROLLING RESPONSIVE TO A COMPARISON OF SAID TEMPERATURE DIFFERENCE, BETWEEN SAID POINTS, WITH A TEMPERATURE DIFFERENCE WHICH IS PERMISSIBLE BETWEEN SAID POINTS FOR THE MEDIUM BEING HEATED, AT THE TEMPERATURE OF THE MEDIUM BETWEEN SAID TWO POINTS. 